What are primordia?

Hyphal knots come first, primordia grow from them, and "pin" is the grower's word for a primordium. GroCycle calls hyphal knots "the first point in a mushroom's development visible to the naked eye", which then "develop into primordia or baby mushrooms" (GroCycle).

Term	What it is
Hyphal knot	A clump of mycelium. In button mushrooms, a fluffy knot 0.5–1 mm across
Primordium	"The earliest recognizable stage of fruitbody development" (North Spore booklet)
Pin or pinhead	A young primordium. In button mushrooms, the pinhead stage has a cap under 5 mm

The button mushroom sizes come from a 2020 review, which says the phases "are difficult to discern" (Molecules, via PMC). Sources don't even agree on the order of names. A UC Cooperative Extension diagram runs hyphal knot, pinhead, primordium (UC Cooperative Extension), while UF/IFAS has primordia developing into the "pinhead" stage (UF/IFAS). For growers, primordium and pin mean the same thing.

Primordia are the first sign that a flush has started, and more form than grow. In button mushrooms, "the number of primordia that develop in the casing soil far exceeds the number of mushrooms produced" (Molecules, via PMC). GroCycle says the fungus picks "the healthiest, most viable pins" to grow (GroCycle). By our reading, a few pins that stay small are normal; a whole cluster drying off is not.

Primordia need the highest humidity of the grow and a gentle hand.

• Humidity: North Spore lists 95–100% RH for blue oyster primordia and 85–95% for the growing mushrooms, and lowers humidity 5–10% once pins have formed (North Spore booklet).
• Misting: mist from a distance with a fine spray. GroCycle warns that "direct, close proximity misting" damages "the fragile primordia" (GroCycle).
• Fresh air: keep CO2 down with fresh air exchange. For lion's mane primordia, North Spore lists 500–700 ppm (same booklet).

Primordia are most often confused with mold. Our contamination guide's look-alikes table shows pins as small knots, while raised fluff that greys or fills the container within a day or two suggests cobweb mold. Primordia that stop growing are aborts, and North Spore says pins usually abort from low humidity (North Spore help center).

Hypha keeps photos organized by grow, so you can document a block's primordia and watch them develop from one day to the next.